The present invention relates to a silica glass optical material, a method for producing the same, and to a projection lens; in further detail, it relates to a silica glass optical material for a projection lens to be used in vacuum ultraviolet radiation lithography using radiation from 155 to 195 nm in wavelength (mainly, in an aligner for producing ultra high density integrated circuits using an excimer laser or an excimer lamp as the light source), to a method for producing the same, and to the projection lens.
Conventionally, ultraviolet radiation using a mercury vapor lamp, such as g-line and i-line, had been used as the light source for producing patterns of integrated circuits on a silicon wafer. However, as the semiconductor devices become finer, the aforementioned g-line and i-line found limits in resolution. Accordingly, excimer lasers which emit radiation with shorter wavelength attracted attention, and a photolithographic device using KrF excimer laser (248 nm) has been developed and put into practice. However, a higher degree of integration of the semiconductor devices is expected to be achieved in the near future, and this requires a light source capable of producing fine patterns with line widths of 0.1 xcexcm or still finer. As the light sources capable of satisfying the requirements above, there can be mentioned high power output vacuum ultraviolet radiation from 155 to 195 nm in wavelength. Thus, efforts are devoted mainly to the development of an ArF excimer laser (193 nm), as well as an ArCl excimer laser (175 nm), a F2 excimer laser (157 nm), etc. However, because the high power output vacuum ultraviolet radiation are far higher in power as compared with those used conventionally in photolithographic devices, the optical materials subjected to the irradiation may suffer abrupt damage such as a drop in transmittance, an increase in refractive index, generation of strain, generation of fluorescence, occasional generation of micro-cracks, etc., and this may make the material practically unfeasible concerning their function as a projection lens.
In the light of such circumstances, the development of an optical material that suffers less damage by the irradiation of the aforementioned high power output ultraviolet radiation emitted by an excimer laser or an excimer lamp has been keenly demanded.
As a material that satisfies the aforementioned requirements, there is known a material disclosed in JP-A-Hei6-227827. More specifically, the optical material disclosed in the publication above is a transparent quartz glass produced by heating a porous quartz glass body formed by depositing fine quartz glass particles obtained by flame hydrolysis and growing it, characterized in that the transparent quartz glass contains 10 ppm or less of OH, 400 ppm or more of a halogen, and that it contains hydrogen.
As an optical material that meets to the demands above, the present inventors have proposed, in JP-B-Hei6-48734 (the term xe2x80x9cJP-B-xe2x80x9d as referred herein signifies xe2x80x9can examined published Japanese patent applicationxe2x80x9d), an optical material for laser radiation having a gaseous hydrogen concentration of at least 5xc3x971016 (molecules/cm3) or higher and an OH group concentration of 100 wtppm or higher. Furthermore, in JP-B-Hei6-27013, the present inventors proposed a synthetic silica glass optical body having a hydrogen gas concentration of at least 5xc3x971016 (molecules/cm3) or higher, an OH group concentration of 100 wtppm or higher, and substantially free from distribution in fluctuation of refractive index by canceling out the distribution in fluctuation of refractive index based on the concentration distribution of OH groups by the distribution in fluctuation of refractive index based on the fictive temperature.
The conventional silica glass optical materials above were satisfactory when used with an excimer laser or an excimer lamp emitting radiation from 195 to 250 nm in wavelength, or with an excimer radiation 195 nm or shorter in wavelength applied to thin members such as photomasks.
However, in drawing circuit patterns using the excimer radiation above, a projection lens made of silica glass is used, and such a projection lens becomes a large optical element exceeding a size 200 mm in diameter and 30 mm in thickness. Thus, if the silica glass optical material as above is used in a projection lens, non-uniform distribution likely occurs in the concentration of hydrogen molecules and OH groups, and this leads to inferior optical characteristics ascribed to the change in transmittance and refractive index. If OH groups should be contained in the silica glass optical material in such a high concentration as to exceed 100 wtppm, the durability becomes inferior due to a drop in the initial transmittance in the vacuum ultraviolet region. That is, the optical material proposed in the published patent application suffered problems of low initial transmittance in the wavelength region of from 155 to 195 nm and of insufficient durability.
The optical material disclosed in JP-A-Hei6-227827 utilizes halogen, however, among the halogens, Cl and the like are apt to generate defects upon irradiation by ultraviolet radiation, and it suffers a serious problem of deteriorating the performance of the optical material such as transmittance in the targeted spectral region.
It is an object of the present invention is to provide a silica glass optical material having a high initial transmittance with respect to vacuum ultraviolet radiation in a wavelength region of from 155 to 195 nm, a high precision, high durability, and an excellent homogeneity.
It is a further object of the invention to provide a method for producing the silica glass optical material.
It is another object of the invention to provide a projection lens made from the silica glass optical material.
The present inventors have intensively conducted studies, and it has been found that there can be obtained a synthetic silica glass optical material having high transmittance, high homogeneity, and an excellent durability, by increasing the purity of the optical material than that disclosed in the published patent application above while controlling the concentration of the OH groups and the hydrogen molecules in a certain range, and by making the concentration distribution thereof uniform while particularly selecting fluorine from the halogens and controlling the concentration of fluorine to a specific range with the distribution thereof being axially symmetrical. The present invention has been accomplished based on these findings.
The problems above can be solved by one of the compositions described in (1) to (14) below.
(1) A silica glass optical material for a projection lens to be used in vacuum ultraviolet radiation lithography using radiation from 155 to 195 nm in wavelength, wherein, said silica glass optical material is of ultrahigh purity, contains from 1 to 10 wtppm of OH groups, from 100 to 10,000 wtppm of F, and from 1xc3x971017 to 1xc3x971019 molecules/cm3 of H2, and has a distribution in concentration of F that is axially symmetrical to the central axis.
(2) A silica glass optical material of (1) above, wherein the value of F/OH is in a range of from 50 to 1000.
(3) A silica glass optical material of (1) or (2) above, wherein the material is of ultrahigh purity, i.e., containing a maximum of 1 wtppb each of Li, Na, and K, a maximum of 0.5 wtppb each of Ca and Mg, and a maximum of 0.1 wtppb each of Cr, Fe, Ni, Mo, and W.
(4) A silica glass optical material described in one of (1) to (3), wherein the material contains a maximum of 1xc3x971017 molecules/cm3 of H2O.
(5) A silica glass optical material described in one of (1) to (4), the distribution in concentration of F that is axially symmetrical to the central axis is such that the concentration gradually increases or decreases from the central portion towards the outer peripheral portion of the silica glass optical material.
(6) A silica glass optical material described in (5), wherein the curve expressing the distribution in concentration of F that is axially symmetrical is approximately a parabola or a quadratic curve.
(7) A silica glass optical material described in one of (1) to (6), wherein the fluctuation range in concentration of F, xcex94F, is within 50 wtppm.
(8) A silica glass optical material described in one of (1) to (7), wherein the material contains a maximum of 10 wtppm of Cl.
(9) A silica glass optical material described in one of (1) to (8), wherein the fluctuation range in the concentration of H2, xcex94H2, is in a maximum of 1xc3x971017 molecules/cm3.
(10) A silica glass optical material described in one of (1) to (9), wherein the fluctuation range in refractive index, xcex94n, in a range of not higher than 2xc3x9710xe2x88x926.
(11) A silica glass optical material of (1) to (10), wherein the amount of strain is not more than 1 nm/cm.
(12) A projection lens for use in vacuum ultraviolet radiation lithography using a silica glass optical material as described in one of (1) to (11).
(13) In a method for producing a silica glass optical material for projection lens to be used in vacuum ultraviolet radiation lithography using radiation from 155 to 195 nm in wavelength, said method for producing a silica glass optical material comprises
producing a white colored soot body having an approximately cylindrical shape and containing OH groups by means of flame hydrolysis of a silicon compound,
performing fluorine doping treatment to the resulting soot body by subjecting it to a heat treatment in a fluorine-containing gaseous atmosphere to obtain a soot body containing OH groups and fluorine,
vitrifying the resulting body to obtain a transparent body by heating it in a reduced pressure atmosphere,
performing zone melting rotary stirring treatment by flame heating said transparent glass body sequentially from one end portion to the other end portion to thereby realize a distribution in fluorine concentration as such to be symmetrical to the rotation axis,
preparing a molding frame having a cylindrical molding cavity,
melting and molding said transparent glass body into an approximately cylindrical shape by placing said glass body inside said molding frame in such a manner that the axis of rotation may be superposed to the central axis of said molding frame,
a removing strain by annealing treatment, and finally performing gaseous hydrogen doping by applying heat treatment in a gaseous atmosphere containing hydrogen molecules.
(14) A method for producing a silica glass optical material of (13) above, wherein the annealing treatment is performed in a gaseous atmosphere containing hydrogen molecules, thereby performing simultaneously the annealing treatment and the hydrogen gas doping treatment.